Image analysis of pathologic specimens with emphasis on quantitation of cell proliferation has been underway over the past two years. The focus has been on utilizing state-of-the-art computer software to quantitate cell turnover in liver and lung tissues following exposure to genotoxic and nongenotoxic carcinogens. Development of immunohistochemical procedures for demonstrating S-phase nuclei following administration of bromodeoxyuridine offers great promise as a useful technique. In addition. new methods for demonstrating proliferating cell nuclear antigen have recently been established in our laboratory. The new techniques are being applied to tissues of mice exposed *o methylene chloride by inhalation for varying periods of time, including a two-year carcinogenesis study. Standard morphometric techniques are being used to chart development and growth of preneoplastic lesions and tumor progression. This work will continue for the next two years.